After reading many posts I decided to switch the dizzy vacuum from ported to full on the carb just to see if there is any difference. (I've been having some tuning issues but that's another story). The carb is 750cfm Edelbrock (1407) on top of a 454. I was going to take the car for a test drive but didn't due a strange noise I heard while idling. I identified the source of the noise. It's the dizzy or the vacuum diaphragm on the dizzy. When I disconnect the vacuum hose from the dizzy vacuum diaphragm the noise goes away. When I reconnect it, the noise starts up again. Almost like a “flute” noise. (see link for short video) I am wondering if the noise was there all along but given I had the dizzy on ported vacuum (per my understanding, no vacuum at idle) I didn't hear it. The noise couldn't be heard once you start driving (per my understanding, when the ported vacuum would kick in) as it gets pretty loud in the car. I wonder if the diaphragm is shot? Thanks in advance...

http://www.mattlekawa.com/pictures/camaro/dizzyNoise.avi

Wow! That's annoying!

To test the diaphragm, 1) attach longer hose between mouth and vac can. 2) suck.
Alternate method, smoke cigarette and blow smoke into can and see if it comes out. Good luck!:D

Wow! That's annoying!

To test the diaphragm, 1) attach longer hose between mouth and vac can. 2) suck.
Alternate method, smoke cigarette and blow smoke into can and see if it comes out. Good luck!:D

So via the first method when sucking on the hose that's attached to the diaphragm, should I be looking for 100% seal or ??? Interesting alternate method...do I have to inhale? :D

You are correct. If it doesn't hold vacuum 100% the diaphragm is bad.
"Alternate, alternate method"- OK to substitute other smokable organic matter for cigarette.:D

OK, I ran slightly more scientific diagnosis. (Couldn’t find any cigs or other organic matter)

First I hooked up a straight vacuum gauge to the vacuum port on the carb. At idle it's pulling 17 inches. Then I put a 'T' into the line so I could hook one end of the line to the carb straight vacuum port, the other to the diaphragm with the vacuum gauge in the middle. At idle it's pulling a lousy 4 inches and making the flute noise. Just to confirm, I yanked the end of the line of the diaphragm, plug it with my finger and the gauge reads 17 inches again. Of course the flute noise stops.

I think this concludes the test and I need a new advance diaphragm. Not to familiar with these. Do they came rather generic or are they specific per various applications?

They are stamped with a specific designation. If yours was working properly, replace it with the same one. To remove the vac can, you need to remove the dist cap and rotor. Then remove the screws. You will probably have to suck on a piece of vac line connected to the vac can in order to move the can enough to remove the back screw. If the diaphram is ruptured, you won't be able to move it, you will have to figure out another method.

Vac cans and specs are listed in JohnZ's article about vac advance and timing.

alan

I was able to get it off no problem. Looks like it has AR-10 stamped on it. I'll start researching that... Thanks.

http://www.mattlekawa.com/pictures/camaro/distAdvVacCan.jpg

Found JohnZ article...must read for anyone wanting to understand vacuum advance timing... Here is the link:
http://www.camaros.net/forums/showthread.php?t=150344

OK..so based on what I'm finding the AR10 was used only on mid 70's small blocks (305/350) and it has max of 9 deg @ 11-13 in. of vac. On the other hand the AR1 was used on mid 70's big blocks (454) and has a max of 9 deg @ 6-8 in. of vac. So same amount of max advance but at different levels of vac. So why would my mid 70's 454 have an AR10? Don't answer that cause considering the "stuff" I have found on this car, anything is possible. Given I am pulling 17 in. @ idle, the question would now be:

Would my setup be better of with AR1 or AR10?

As the engine builds RPM's the vac drops. So with AR1 I would keep the advance a little longer as the vac will need to drop below 6-8 in. for the diaphragm to back off. The AR10 would keep the advance a little shorter as the vac will need to drop below 11-13 in. for the diaphragm to back off. Not sure how this complements the centrifugal advance via the springs. Any chance JohnZ is reading this?

Here is an ACCEL adjustable vacuum advance I found for GM HEI. The advance can be adjusted 0-21 degrees @6-7 inch of vac. It is based on a 24 degree mcchanical advance at the crank. Has anyone used this?

http://www.summitracing.com/parts/ACC-31035/

Well, I have no helpful input. Just going to say that I would stick with the same higher vacuum one.
Both the other two give you full advance at a lower vacuum level, they may cause pinging.
You could try it but if you get pinging you might have to switch to the AR10

OK, I ran slightly more scientific diagnosis. (Couldn’t find any cigs or other organic matter)

First I hooked up a straight vacuum gauge to the vacuum port on the carb. At idle it's pulling 17 inches. Then I put a 'T' into the line so I could hook one end of the line to the carb straight vacuum port, the other to the diaphragm with the vacuum gauge in the middle. At idle it's pulling a lousy 4 inches and making the flute noise. Just to confirm, I yanked the end of the line of the diaphragm, plug it with my finger and the gauge reads 17 inches again. Of course the flute noise stops.

I think this concludes the test and I need a new advance diaphragm. Not to familiar with these. Do they came rather generic or are they specific per various applications?

Excellent test procedure. Definately a bad diaphram. I used a crane aftermarket vacuum diaphram. It has an adjustment inside the vacuum port to set the vacuum level for full advance. It also has a little notched washer that fits under a mounting screw to limit the full advance. If your motor is stock, an OEM or aftermarket replacement will probably work fine. But if you have a different cam, compression ratio you'll like the adjustability of the crane unit.

After reading tons of posts regarding timing, I have decided to go with the an adjustable can/diaphragm. I am also going to order new springs and weights as the ones I have now are worn and a bit rusty.

As the engine builds RPM's the vac drops. So with AR1 I would keep the advance a little longer as the vac will need to drop below 6-8 in. for the diaphragm to back off. The AR10 would keep the advance a little shorter as the vac will need to drop below 11-13 in. for the diaphragm to back off. Not sure how this complements the centrifugal advance via the springs. Any chance JohnZ is reading this?

Don't worry about when it backs off - what matters is when it starts pulling in, and at what level of manifold vacuum it's fully-deployed.

The "two-inch rule" I've always used says that the can must be fully deployed at a vacuum level at least 2" Hg. less than normal idle manifold vacuum, to maintain a stable idle.

What you're after is a can that starts pulling anywhere between 5"-9" Hg. and is fully deployed at least 2" Hg. less than your 17" Hg. idle vacuum; I'd shoot for one that's all in by 12"-14" Hg.

I'm not too familiar with the "AR-" HEI cans (their bracket is longer than those for point distributors I work on), but the AR-12 is a good all-around choice, with the AR-15 and AR-23 having almost the same specs. They all provide about the same advance, and are fully deployed at a vacuum level well below your idle vacuum.

:beers:

John, thanks for info. I got everything put back together and will be dialing the advacne over the weekend.

OK, I did some tuning and here are the results with question. Please see the thread below as I had a another dizzy thread going so I am trying to consolidate the two together...

http://www.camaros.net/forums/showthread.php?t=162942